In-vehicle DC/DC converters require a transformer and a coil, which operate with a large current. Such high power inductance devices use ferrite as a magnetic core material because they are expected to operate at high frequency ranges. However, ferrite is likely to be magnetically saturated because its saturation magnetic flux density is not so high. Therefore, a large magnetic path cross-sectional area must be ensured, which necessarily causes a ferrite magnetic core to be upsized and increases a heating value due to a large current flowing through a winding wire.
As is well known, the temperatures of various electronic devices increase with heat generated when the devices are operated, and components are damaged or degraded if such increases in the temperatures greatly exceed the heat resistant temperatures of materials forming the components. Large inductance devices operating with a large current have a high heating value and thus are subjected to measures against heat (see, for example, Patent Literature 1, or the like) in which a part of a ferrite magnetic core is brought into contact with a heat radiation structure such as a housing, a printed board, and a heat radiation plate either in a direct manner, in an indirect manner through a material such as an adhesive, or in a pseudo-contact manner with a minute air gap placed therebetween to release most of generated heat to the heat radiation structure via the ferrite magnetic core.
With such a method, a temperature on the side of the cooling surface (on the side of the surface opposing the heat radiation structure) of the ferrite magnetic core is lowered. However, because ferrite typically has low heat conductivity, the temperature of a part away from the cooling surface is not lowered as much as the side of the cooling surface and thus a considerable temperature difference occurs. The larger the ferrite magnetic core, the longer the length of a heat flow path becomes. Therefore, the heat resistance of the ferrite magnetic core becomes high, which increases the temperature difference between the part away from the cooling surface and a part near the cooling surface. Particularly for the high power large inductance device, it is difficult to prevent a temperature increase in the part away from the cooling surface due to its high heating value.
Next, there is typically a problem in mass-producing large ferrite magnetic cores with excellent dimensional accuracy because ferrite is a sintered body. The larger a ferrite magnetic core, the more the deformation of the ferrite magnetic core such as warpage is likely to occur when the ferrite magnetic core is burnt. In an extreme case, cracks, or the like may occur in the ferrite magnetic core, which causes the degradation of a manufacturing yield. In view of this, there has also been proposed a method (see, for example, Patent Literature 2) in which a large ferrite magnetic core is made of an aggregate of a plurality of relatively small cores. With this method, the plurality of ferrite cores are arranged side by side in a close contact state such that magnetic paths are parallel to each other, thereby obtaining a required magnetic path cross-sectional area.
However, when the ferrite cores are bonded together in the close contact state, they are made to collide with each other by excessive stress, vibration, or the like resulting from heat deformation caused when they are operated. Consequently, problems such as core cracks and in an extreme case core breaking may occur, which leads to a lack in reliability.
Accordingly, it is requested that a problem in the temperature increase of a ferrite magnetic core accompanied by a high power capacity and a problem such as the degradation of productivity and reliability accompanied by the upsizing of the ferrite magnetic core be solved by conventional technologies at the same time. However, such problems still remain.